1. Field of the Invention
This invention relates to processes for recovering useful products efficiently from plant materials by solvent and water extraction. More specifically, the invention relates to the processing of fibrous plant materials such as guayule shrub and guayule-like material to recover and isolate rubber, resins, water-solubles and bagasse as herein defined, and related materials by either water flotation or solvent extraction or a combination of both.
2. Description of the Prior Art
The usual natural rubber of commerce is obtained by tapping the Hevea rubber tree (Hevea brasiliensis), collecting its latex, and coagulating the latex to obtain rubber. Neither Hevea nor any other similar rubber producing tree can be practically grown in the United States. Future green plant production of rubber and hydrocarbons in the United States will probably involve guayule and guayule-like plants and require extraction, recovery and isolation processes more like those applicable to guayule (Parthenium argentatum) than those applicable to Hevea.
Guayule is one of many rubber producing plants. It has served as an important source of rubber in the past and interest in it has recently revived. The prehistoric native process for extracting rubber from guayule was to chew pieces of shrub, expectorate fibrous matter and retain gummy matter until a mass of resinous rubber accumulated in the mouth. Consolidation of many chewing-gum sized pieces provided enough material for rubber balls and other articles.
It is significant that the most modern process for extracting rubber from guayule is that, for example, practiced in a pilot plant at Saltillo, Coahuila, Mexico, and described in detail by the National Academy of Sciences 1977 booklet "Guayule: An Alternative Source of Natural Rubber."
This process involves mastication of ground guayule shrub, which is effected by wet-milling with a rotating disc (pulping) mill, to recover the resinous rubber in the form of "worms" which are separated from fibrous matter by flotation in water. Antecedent to wet-milling, lush guayule shrub is parboiled to coagulate the rubber latex and aid in removing leaves, then hammermilled to a size suitable for wet grinding in the pulping mill. Subsequent to the wet-milling and separation of bark and wood, the recovered rubber is deresinated by acetone extraction and purified by dissolving in cyclohexane and filtering or centrifuging. These main processing steps and several auxiliary operations make up the current complicated separation process for winning rubber from guayule shrub. For the sake of clarity in presenting data, the preceding described process will be referred to as the "Saltillo" process.
The Saltillo process is essentially the same as that described in U.S. Pat. No. 2,434,412. It is also similar to the process taught in U.S. Pat. No. 2,459,369 except that deresination of the rubber results in a lower recovery of resin than in the patented process of deresinating the whole plant material; this patent does, however, indicate that it would be optional to process defoliated shrub. This statement should be considered in the light of the statement in U.S. Pat. No. 4,159,903 (col. 4, lines 25-27) that the entire plant, except the leaves, must be processed to extract the rubber.
3. General Background
Processing of whole guayule shrub has definite economic and energy-conservation benefits. For example, the processing costs and energy requirements for parboiling and defoliating of guayule shrub are eliminated.
Additional benefits of applicants' invention will be more apparent if one realizes that guayule shrub contains not only a valuable rubber component but also a sugar fraction generally described as containing sugars and polysacchrides as well as a resin fraction containing terpenes and glyceride-esters. Again, for the sake of clarity, the sugar and polysacchride fraction will be referred to as the water-soluble fraction to distinguish it from the resin fraction which will be defined based on solubility characteristics; therefore, the resin fraction is defined as that portion of guayule shrub which is soluble in a deresinating solvent such as acetone or other solvent herein disclosed as being equivalent to acetone.
A really efficient guayule processing procedure should recover all of the inherently valuable components of guayule shrub. Applicants will endeavor to specifically disclose the advantages of their invention over the prior art relative to recovery of all of the valuable components of guayule shrub in an economical and energy-efficient manner.
In general, to the Saltillo process will be referred to as the most technologically advanced process of the prior art. The procedure of recovering and isolating guayule rubber by dispersing ground guayule shrub in water and allowing the rubber to float to the surface for subsequent recovery will be referred to as the water-flotation procedure.
The process of extracting rubber directly from the guayule shrub with a hydrocarbon will broadly be referred to as solvent extraction. Solvent extraction of rubber can be effected by either percolation or total immersion. By "percolation" it is meant that solvent is passed through a fixed bed of shrub to effect extraction. By "immersion" it is meant that the shrub is immersed in solvent contained in a suitable vessel and the shrub/solvent mixture can be agitated to effect extraction.
By "whole" guayule shrub it is meant the complete shrub including the roots, base, stem, branches and leaves. It is also intended to also include "pollarded" guayule shrub in the above definition of "whole" guayule shrub.
"Pollarded" guayule shrub is defined as shrub which is harvested by cutting the shrub a few cm above ground level and harvesting the greater portion of the base, branches, stems and leaves. The root system is left in the soil to generate another "shrub". By defoliated guayule shrub is meant the whole guayule shrub (including pollarded shrub) which has been treated to remove essentially all of the leaves.
Other abbreviations used in this specification refer to scientifically accepted notations.
For reasons of convenience and economy, it has been necessary to demonstrate the inventive processes using wild guayule shrub native to Texas. It is believed that the disclosed processes would give essentially the same results with cultivated guayule and guayule-like shrubs of all varieties and of generally higher rubber content.
Those skilled in the art have long recognized that direct solvent extraction of rubber from guayule shrub seems to offer many advantages over water flotation. The primary advantage would be elimination of many processing steps including parboiling and/or pressure cooking; wet-milling; addition of acid, alkali, surfactants, etc.; flotation; water washing; and several drying and purification steps. Subsidiary advantages to be expected from solvent extraction in comparison to water flotation are large savings in process water (guayule grows and is processed in arid regions), possible improved resin yields and lower heavy-metal contamination of the rubber and the provisions for solution-phase purification as an integral component of the operation. However, solvent extraction of rubber from guayule, though often studied, has been found completely impractical on a commercial scale, heretofore. In fact, the earliest industrial techniques for guayule processing were based on solvent extraction but were found impractically difficult and expensive and were replaced by wet-milling techniques, see U.S. Pat. No. 982,373. Solvent extraction is contemplated in U.S. Pat. No. 1,695,676 wherein the cell walls of the guayule shrub are first broken down by penetrating the plant material with high pressure gas and suddenly releasing it to effect an instantaneous expansion. The advantages of this process are diminished by the expense of the requisite high pressure equipment and the inherent inefficiency of a batch-type procedure.
As is well known, a fundamental problem in solvent extraction of rubber from plant materials is that rubber is a high molecular weight polymer which passes through cell walls and membranous tissue with difficulty even when in solution. Thus, to accomplish solvent extraction of rubber, the plant structure has to be very thoroughly disrupted.
U.S. Pat. No. 4,136,131 is directed to the extraction of rubber or rubber-like materials from fibrous plants, such as guayule, wherein said plants are subjected to simultaneous compression and high shear forces under nonaqueous conditions to result in (1) comminuted fibrous matter and (2) rubber-like substances which cohere into a plastic mass; this mass is shaped into particles; said rubber-like substances are then solvent extracted from the particles and then recovered from the solvent.
Additional prior art patents of a general background nature include U.S. Pat. Nos. 1,753,185; 2,744,125; 2,666,317; 2,618,670; 2,572,046 and 2,549,763.
A recent U.S. Pat. No. 4,159,903, is directed to the improved production of high molecular weight polyisoprene rubber by administering to plants, such as guayule, an amine-type growth regulator. In column 4, lines 25-27 it is stated that the natural rubber latex in guayule is found in all parts of the plant but the leaves; it is then stated that the entire plant, except the leaves, must be processed to extract the rubber.